Fuel cells have been proposed as a power source for many applications. One such fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell. PEM fuel cells are well known in the art and include in each cell thereof a so-called “membrane-electrode-assembly” (hereafter MEA) comprising a thin (i.e., ca. 0.0015–0.007 inch), proton-conductive, polymeric, membrane-electrolyte having an anode electrode film (i.e., ca. 0.002 inch) formed on one face thereof, and a cathode electrode film (i.e., ca. 0.002 inch) formed on the opposite face thereof. Such membrane-electrolytes are well known in the art and are described in such U.S. patents as U.S. Pat. Nos. 5,272,017 and 3,134,697, as well as in the Journal of Power Sources, Volume 29 (1990) pages 367–387, inter alia. In general, such membrane-electrolytes are made from ion-exchange resins, and typically comprise a perfluoronated sulfonic acid polymer such as NAFION™ available from the E.I. DuPont de Nemours & Co. The anode and cathode films, on the other hand, typically comprise (1) finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive material (e.g., NAFION™) intermingled with the catalytic and carbon particles, or (2) catalytic particles, sans carbon, dispersed throughout a polytetrafluoroethylene (PTFE) binder. One such MEA and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993, and assigned to the assignee of the present invention.
The MEA is sandwiched between sheets of porous, gas-permeable, conductive material, known as a “diffusion layer”, which press against the anode and cathode faces of the MEA and serve as (1) the primary current collectors for the anode and cathode, and (2) mechanical support for the MEA. Suitable such primary current collector sheets comprise carbon or graphite paper or cloth, fine mesh noble metal screen, and the like, through which the gas can diffuse, or be driven, to contact the MEA underlying the lands, as is well known in the art.
The thusly formed sandwich is pressed between a pair of electrically conductive plates which serve as secondary current collectors for collecting the current from the primary current collectors, and for conducting current between adjacent cells internally of the stack (i.e., in the case of bipolar plates), and externally of the stack (in the case of monopolar plates at the ends of the stack). The secondary current collecting plates each contain at least one active region including a so-called “flow-field” that distributes the fuel cell's gaseous reactants (e.g., H2 or O2/air) over the surfaces of the anode and cathode. The flow-field includes a plurality of lands which engage the primary current collector and define therebetween a plurality of grooves or flow-channels through which the gaseous reactants flow between a supply manifold in a header region of the plate at one end of the channel and an exhaust manifold in a header region of the plate at the other end of the channel.
The pressure differentials (1) between the supply manifold and the exhaust manifold, and (2) between adjacent flow channels, or segments of the same flow channel, are of considerable importance in designing a fuel cell. Serpentine channels have been used to achieve desired manifold-to-manifold pressure differentials as well as inter-channel pressure differentials. Serpentine flow-channels have an odd number of legs extending, in switchback style, between the supply and exhaust manifolds of the stack Serpentine flow channels use various widths, depths and lengths to vary the pressure differentials between the supply and exhaust manifolds, and may be designed to drive some reactant gas trans-land between adjacent channels, or between adjacent segments of the same channel, via the current collecting diffusion layer in order to expose the MEA confronting the land separating the legs to reactant. For example, some gas can flow from an upstream leg of a channel (i.e. where pressure is higher) to a parallel downstream leg of the same channel (i.e. where the pressure is lower) by moving the gas through the diffusion layer engaging the land that separates the upstream leg from the parallel downstream leg. Non-serpentine flow-channels have been proposed that extend more or less directly between the supply and exhaust manifolds, i.e. without any hairpin/switchback-type turns therein, and hence in shorter lengths than the serpentine flow-channels. Pressure differential management is more difficult with non-serpentine flow-channels than with serpentine flow-channels.
The present invention is directed to a PEM fuel cell flow-field that offers significant design flexibility in achieving desired pressure differentials between the supply and exhaust manifolds, between adjacent flow-channels, and/or between segments of the same flow-channel. The invention utilizes flow-restrictions strategically located throughout the flow-field to achieve the desired pressure differentials, and is particularly useful with non-serpentine flow-channels.